Method of tablet enrobing

ABSTRACT

An ingestible tablet ( 10 ), e.g. of a medicament, is enrobed to produce a tamper-evident coating by vacuum forming a film ( 40, 46 ) of material, preferably hydroxypropyl methyl cellulose, onto the surface of the tablet.

FIELD OF THE INVENTION

This invention concerns tablet enrobing, that is coating ingestibletablets, e.g. of a medicament, vitamin, dietary supplement etc, withsuitable ingestible material so that the tablets are tamper-evident,i.e. any attempt to tamper with the tablet e.g. by adulterating thecontents will result in damage to the coating that is readily visuallyapparent. The invention is applicable to other solid forms of medicamentetc, e.g. caplets and capsules as well as tablets, but for simplicityall such forms will generally be referred to herein as tablets.

BACKGROUND TO THE INVENTION

As a safety precaution, it is nowadays becoming increasingly desirableor necessary to provide tamper-evident tablets in addition totamper-evidence packaging for tablets of medicaments etc. It is known toenrobe tablets in gelatin for this purpose by dipping tablets intogelatin solution and allowing the solution to dry to form a coating. Thegelatin solution may be coloured, and it is known to produce dual colourgelatin coatings, e.g. by coating the entire tablet with, say, yellowgelatin and then, after drying, coating half of the tablet with, say,red gelatin. This produces an attractive, tamper-evident tablet.However, the process involves several steps and is time consuming, evenwith single coating processes, as long drying times are involved.Further, problems are associated with use of gelatin in ingestibleproducts as gelatin is an animal-derived material, obtained from thebones and skins of animals such as cattle, and there are increasingconcerns over use of such materials in ingestible products in view offear of animal related diseases such as Bovine SpongiformEnceptialopathy (BSE).

The present invention provides an alternative approach to tabletenrobing not necessarily using gelatin coatings.

SUMMARY OF THE INVENTION

In one aspect, the invention provides a method of enrobing a tablet,comprising vacuum forming a film of material onto the surface of thetablet.

In practising the method of the invention, the tablet and enrobing filmmaterial are exposed to conditions of differential pressure across thefilm, with a vacuum or substantially reduced pressure on the side of thefilm material in the vicinity of the tablet, so that the film materialis caused to deform so as to conform to the external surface of thetablet under the action of the pressure differential, forming askin-tight coating on at least part of the tablet surface, fittingsnugly to the tablet surface. In this way a skin-tight, tamper-evidentfilm wrapping of the tablet may be produced. In order fully to enclosethe tablet in the film material, it may be necessary to perform two ormore vacuum forming steps, with different parts of the tablet in thevicinity of the film material prior to exposure to the pressuredifferential. Vacuum chamber or vacuum bed apparatus, in which thetablet is located on a suitably shaped support and exposed to conditionsof vacuum (or substantially reduced pressure) can be used for vacuumforming. Such apparatus may be based on commercially available vacuumchamber or vacuum bed apparatus. Vacuum forming techniques result in thecoating forming a vacuum-tight pack around the tablet, with theexclusion of air between the coating and tablet, leading to potentiallybetter keeping properties and hence longer shelf life of the enrobedtablet as compared with uncoated tablet.

The film should be of material that is suitable for human consumptionand that has sufficient flexibility and plasticity to be vacuumformable. Some film materials have suitable properties in their naturalcondition, but commonly it will be necessary to pre-treat the filmmaterial so it is vacuum formable. For example, it may be appropriate toexpose the film material to a solvent therefor; for instance, certaingrades of polyvinyl alcohol (PVA) will vacuum form after application ofa small amount of water to the surface thereof or when exposed toconditions of high humidity. A further, generally preferred, possibilityis to use a film of thermoplastic material (i.e. material capable ofdeforming plastically on heating) with the film being heated to be inheat-softened condition prior to being thermoformed by exposure tovacuum. Suitable thermoplastic materials include modified cellulosematerials, particularly hydroxypropyl methyl cellulose (HPMC) andhydroxypropyl cellulose (HPC), polyvinyl alcohol (PVA), polyethyleneoxide (PEO), pectin, alginate, starches, and modified starches, and alsoprotein films such as soya and whey protein films. The currentlypreferred film material is HPMC. Suitable film materials arecommercially available.

When using film of thermoplastic film, the film is typically heatedprior to application to the tablet (and so usually prior to exposure tothe differential pressure conditions), so that the film is inheat-softened deformable condition. This can be achieved by exposing thefilm to a source of heat, e.g. an infra red heater, infrared lamps, aheated plate, a hot air source etc.

The film material may include optional colourings, e.g. in the form offood dyes such as F D and C yellow number 5, and/or optionalflavourings, e.g. sweeteners, and/or optional textures etc in knownmanner.

The film material typically includes a plasticiser to give desiredproperties of flexibility to the film in known manner. Materials used asplasticisers include alpha hydroxy acids such as lactic acid and saltsthereof, diacetin, triacetin, propylene glycol, glycerin or mixturesthereof. A typical thermoplastic film formulation is HPMC 77% by weight,plasticiser 23% by weight.

The film suitably has a thickness in the range 20 to 200 microns,conveniently 50 to 100 microns, e.g. about 80 microns, with appropriatefilm thickness depending on factors including the size and form of thetablet.

The method of the invention conveniently involves forming two separate,overlapping part (generally half) coatings on the tablet of the filmmaterial. Thus the method preferably involves first coating part(generally half) of the tablet, removing remaining film material notcoated on the tablet, e.g. by cutting, then coating the remaining part(generally half) of the tablet, with overlapping portions of the twocoatings sealed together to provide a sealed complete enclosure for thetablet, and again removing remaining surplus film material not coated onthe tablet. It may be necessary to apply adhesive material or gluebetween the overlapping film coatings, e.g. to the surface of one orboth of the film layers, to ensure formation of an effective sealtherebetween and to make the enrobed tablet tamper-evident. The adhesivematerial conveniently has the same composition as the film, but with agreater proportion of plasticiser, e.g. 93% to 98% by weightplasticiser, so as to provide a less viscous material. The adhesivematerial may be applied, e.g. by use of a roller, spraying etc. Atypical adhesive formulation, with % representing % by weight, is HPMC4%, lactic acid 77%, water 19%.

The tablet conveniently includes a generally cylindrical side wallportion, with the two half coatings overlapping on this side wall.Tablets of circular symmetrical form with a circular cylindrical sidewall are very common, but other forms eg generally oblong and oval,again including a cylindrical side wall, are also known.

It may also be advantageous or desirable to apply adhesive material,e.g. as described above, to the surface of the tablet prior to coatingto promote adhesion of the film thereto. Again this may be achieved e.g.by use of a roller, spraying etc.

A plurality of tablets in an array may conveniently be coatedsimultaneously, using a suitably large sheet of film material.

In a further aspect, the invention provides a tablet enrobed by themethod of the invention.

The invention will be further described by way of illustration withreference to the accompanying drawings, in which:

FIGS. 1 to 9 illustrate schematically enrobing of a tablet by a methodin accordance with the invention using a split vacuum chamber, withFIGS. 1 to 5, 7 and 8 being sectional views, FIGS. 6 and 9 beingperspective views on an enlarged scale and FIGS. 6A and 9A showingdetails of FIGS. 6 and 9, respectively, on a further enlarged scale.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 9 illustrate schematically a method of enrobing a tablet inaccordance with the invention, using a thermoplastic film in a vacuumforming technique. In practice an array of a plurality of tablets willgenerally be coated simultaneously, but for simplicity only one tablet10 is shown in these Figures.

As shown in FIG. 1, tablet 10, is of circular symmetrical form andincludes a generally circular cylindrical side wall portion 12 and twosimilar part-spherical upper and lower portions 14 and 16.

In FIG. 1, tablet 10 is shown located on a platen or support 18 whichincludes a recess 20 shaped to be complementary to tablet lower portion16 (and tablet upper portion 14).

The support 18 and tablet 10 are located in a split vacuum chamber 22 ofgenerally conventional construction, as shown in FIG. 2. The chamber isin the form of a sealed generally cuboid box, and comprises an upperchamber portion 24 and a lower chamber portion 26 that fit sealinglytogether, with a circumferential seal 28 therebetween. Upper chamberportion 24 includes a vacuum port 30, and lower chamber portion 26includes a vacuum port 32, but the chamber is otherwise enclosed andsealed with respect to the exterior.

The support 18 is located on two elongate protrusions 34, 36 extendingupwardly into the chamber cavity from lower chamber portion 26. Aheatable plate shown schematically at 38 is located in upper chamberportion 24. A sheet 40 of hydroxypropyl methyl cellulose (HPMC) film 80microns thick is located between the upper and lower chamber portions,trapped and secured in position between the seal 28 and chamber portionsand extending outwardly therefrom. The HPMC film comprises 77% by weightHPMC, 23% by weight lactic acid plasticiser.

In use of the apparatus, plate 38 is heated to a temperature in therange 150 to 200° C., typically about 180° C. A vacuum is then drawn inthe upper chamber, by connecting vacuum port 30 to a vacuum pump, withvacuum port 32 being left open to the atmosphere. The resulting pressuredifferential has the effect of drawing film 40 into contact with thelower surface of heated plate 38, as shown in FIG. 3.

A vacuum is then drawn in the lower chamber, by connecting vacuum port32 to a vacuum pump, with the vacuum in the upper chamber beingmaintained. When the lower chamber is fully evacuated and the film 40 isat the correct temperature for thermoforming due to the effect of heatedplate 38 (generally after about 5 seconds in contact with plate 38), airis readmitted to the upper chamber by opening vacuum port 30 toatmosphere. The resulting pressure differential has the effect ofdrawing film 40, which is in heat-softening condition, down onto theupper and side surfaces of the tablet 10 as shown in FIG. 4. The absenceof air in the lower chamber allows the film to conform precisely to thecontours of the tablet.

When the vacuum forming is complete (generally after about 10 seconds)air is readmitted to the lower chamber and the vacuum chamber is opened.

The vacuum formed web of HPMC film and tablet (retained in the web) isremoved from the chamber on platen 18.

The tablet is cut out from the film web using a close-fitting hollowcylindrical blade 42, as illustrated in FIG. 5. This results in ahalf-enrobed tablet, as shown in FIGS. 6 and 6A, with the upper portion14 and part of the cylindrical side wall portion 12 coated with film 44to a point slightly below the central plane of the tablet, as shown bestin FIG. 6A.

The half-enrobed tablet is put back on the platen 18 in invertedposition, with the enrobed part in contact with recess 20, as shown inFIG. 7.

The platen 18 is returned to the vacuum chamber, with a further sheet 46of the HPMC film located between the upper and lower chamber portions,in like manner to sheet 40 as described above. The vacuum formingprocess, as described above with reference to FIGS. 3 and 4 is repeated,and the resulting vacuum formed web of HPMC film and tablet removed fromthe chamber on platen 18.

The tablet is cut out of the film web using a close-fitting hollowcylindrical blade 48 of slightly larger diameter than blade 42, asillustrated in FIG. 8. This results in the tablet being fully enclosedand enrobed by the two layers of film 44 and 46, with a circumferentialoverlapped seal as shown in FIGS. 9 and 9A.

In order to achieve an effective seal between the overlapping filmlayers, glue is applied to at least one of the overlapping surfaces(e.g. to the outer surface of the cylindrical portion of film 44 coatingthe tablet side wall portion 12) after the first vacuum forming processand before the second vacuum forming process. The glue is of the samechemical composition as the HPMC film, but with a higher proportion ofplasticiser, comprising HPMC 4% by weight, lactic acid 77% by weight andwater 19% by weight. The glue is conveniently applied by use of a rolleror by spraying.

In a modification of the above described apparatus and method, heatedplate 38 is replaced by an array of infra red lamps in the upper chamberportion 24. In use of the apparatus a vacuum is drawn in both the upperand lower chamber portions 24 and 26 by connecting the vacuum ports 30and 32 to a vacuum source. By maintaining equal pressures in both halvesof the vacuum chamber the film sheet 40 is held in position whilst beingheated by absorption of infra red radiation from the infra red lamps.This condition is maintained until the film is at the correcttemperature for thermoforming. Air is then readmitted to the upperchamber portion 24 by opening vacuum port 30 to atmosphere. Thismodification is currently generally thought preferable to the heatedplate version.

1. A method of enrobing a tablet comprising the steps of: providing atablet in a first fixed position; providing a thermoformable film;heating the thermoformable film to a heat-softened deformable condition;at the first fixed position, moving the thermoformable film by vacuumdirectly onto the contours of a portion of the tablet while maintainingthe tablet in the first fixed position; at a second fixed position,moving the thermoformable film by vacuum directly onto the contours ofanother portion of the tablet while maintaining the tablet in the secondfixed position, thereby forming a skin-tight coating on the tablet. 2.The method according to claim 1, wherein the film material ishydroxypropyl methyl cellulose.
 3. The method according to claim 1,wherein the film has a thickness in the range of 20 to 200 microns. 4.The method according to claim 1 wherein the film on the tablet is formedfrom two separate, overlapping coating parts.
 5. The method according toclaim 4, comprising applying adhesive material between the overlappingcoatings parts.
 6. The method according to claim 5, wherein the tabletincludes a generally cylindrical side wall portion, with the two coatingparts overlapping on the side wall portion.
 7. The method according toclaim 1, comprising applying adhesive material to the surface of thetablet prior to vacuum forming of the film.
 8. The method according toclaim 1, wherein a plurality of tablets are coated simultaneously.